The way the preferred address mechanism is supposed to work
is this: A server might be exposed via multiple network
interfaces / addresses. The preferred address is the one that
clients should use. If a client uses one of the non-preferred
addresses to initially connect to the server, the server will
include the preferred address in it's initial transport params
back to the client over the original connection. The client
then can make a choice: it can either choose to ignore the
advertised preferred address and continue using the original,
or it can transition the in-flight connection to the preferred
address without having to restart the connection. In the latter
case, the connection will start making use of the preferred
address but it might not do so immediately.

To test this mechanism properly, we should have our server
configured on multiple endpoints with one of those marked
as the preferred. The connection should start on one and preceed
uninterrupted to completion. If the preferred address policy
is "accept", the client will accept and transition to the servers
preferred address transparently, without interupting the flow.

The current test is deficient because the server is only listening
on a single address which is also advertised as the preferred.
While the client should get the advertisement, we're not actually
making use of the preferred address advertisement and nothing on
the connection changes

The ipv6 version of the test is also hanging on some platforms and
it's not entirely clear why yet.

/cc @nodejs/quic

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines